1. Field of the Invention
The present invention relates to a process for the preparation of homo- and copolymers of .alpha.-olefins by the use of catalyst systems whose titanium (IV) component is first reacted with carrier substances such as magnesium alcoholates or magnesium hydroxychlorides in the presence of atactic poly-.alpha.-olefins. The subsequent reduction and activation of the catalyst system, however, are carried out in the absence of atactic poly-.alpha.-olefins. In this way, a catalyst system is obtained which is distinguished by high activity. Homo- and copolymers of .alpha.-olefins prepared by the use of such a catalyst have a particularly fine-grained structure.
2. Discussion of Prior Art
The polymerization of .alpha.-olefins to the corresponding homo- and copolymers by the use of Ziegler-Natta catalysts is widely employed in the production of plastics having different technically desirable properties. Ziegler-Natta catalysts usually comprise a titanium (III) compound, which can be prepared by reduction of titanium (IV) compound or by reaction of a titanium (IV) compound with carrier substances such as magnesium alcoholates or magnesium hydroxychlorides followed by reduction, and an activator. Such catalyst systems are usually obtained by first reducing a titanium (IV) compound with organoaluminum or organomagnesium compounds to a titanium (III) compound or, after reacting the titanium (IV) compound with magnesium alcoholates or magnesium hydroxychlorides, by effecting the reduction with organoaluminum compounds. Trialkylaluminum compounds or alkylaluminum halides, for example, or mixtures thereof may be used as reducing agents. After the dissolved reducing agent has been washed out, an organoaluminum compound of the type mentioned is added to the titanium (III) compound as activator. In the case of organoaluminum compounds which are used both as reducing agent and as activator, a separate reducing step can be dispensed with. The aluminum compound is then added in a single step during the polymerization.
When this general procedure is followed and the titanium (IV) compound is reacted with magnesium alcoholates or magnesium hydroxychlorides and then reduced, catalyst systems are obtained which result in polymers of relatively large particle size. Often, however, polymers of small particle size are required in view of the further processing of the plastic. This is true especially of ultrahigh-molecular polyolefins, that is to say, those having a viscosimetrically determined molecular weight above 500,000, and in particular above 1,000,000. Such polyolefins cannot be processed into shaped articles by the techniques generally used with thermoplastics, such as extrusion or injection molding. They are therefore processed primarily by pressing and sintering. To provide assurance that additives such as colorants, antioxidants and UV stabilizers are uniformly distributed in semifinished products and finished parts, it is then necessary to employ ultra-high-molecular polyolefins in particularly finely divided form.
Since finely divided catalysts result in particulate polymers, there have been many attempts to produce catalysts of small particle size. U.S. Pat. No. 2,968,652, for example, describes a method of comminuting titanium (III) compounds of a particle size of about 25.mu. to a particle size of approximately 0.1 to 5.mu. by subjecting the crystals to ultrasonic treatment in an inert liquid medium. The fine particles so obtained are said to give within a short time higher yields of crystalline polymers than coarser catalyst particles. However, the particle size of polymers produced by the use of these catalysts still is not small enough, and the products therefore have to be subjected to subsequent size reduction. Because of the equipment needed, this method is not suited for industrial use.
Another method is described in German No. 15 95 661. Here titanium chloride particles of an average size of 0.5 to 3.0.mu. obtained by the reduction of titanium tetrachloride in the presence of a rubberlike active-oxygen-atoms-containing hydrocarbon polymer which contains from 0.1 to 1.0 weight percent oxygen and is dissolved in the solvent used in an amount of 0.01 to 1 part per part by weight of titanium tetrachloride is used as a catalyst component for the production of particulate polymers. Recommended are all known rubberlike hydrocarbon polymers, homopolymers or copolymers, such as polyisobutylene, ethylene-propylene copolymers, polyisoprene, polybutadiene, ethylene-propylene-dicyclopentadiene terpolymers as well as styrene-butadiene polymer which have been oxidized by the action of oxygen. Such polymers can be oxidized by grinding for a short time at elevated temperature in an oxygen atmosphere or by prolonged storage in air.
The use of the rubberlike hydrocarbon polymer containing active oxygen atoms results in the formation of a fine-grained catalyst. In practice, however, production of the hydrocarbon polymers poses difficulties. Oxygen absorption occurs by way of the polymer surface and because of the high viscosity of the product is difficult to control reproducibly so that a homogeneous distribution of the oxygen in the polymer is not assured. The same applies to analytical control. Representative sampling for the purpose of determining the oxygen content of the hydrocarbon polymer as a function of the duration of the reaction and the temperature level is possible only to a limited extent. Moreover, it should not be overlooked in this context that the activity of the catalyst can be impaired by the presence of oxygen-containing compounds.
German No. 17 95 197 describes a process for the homo- and copolymerization of olefins by the use of a mixed catalyst formed by the product of reaction of a titanium compound with a magnesium compound (component B), component A being produced by the reaction of magnesium alcoholates with tetravalent halogen-containing titanium compounds.
An advantage is said to be that the products of reaction of magnesium alcoholates with titanium (IV) halogen compounds constitute particularly active supported catalysts.
However, the use of such supported catalysts results in polymers formed of relatively coarse particles.
Thus, there has been a need for a process for the preparation of particulate polymers which does not have the drawbacks described.